To put things back on track, I didn’t have any issue with my HP Photosmart printer after the upgrade. In fact it used to have a connection problem once in a while after turning it on but after the upgrade that behavior seems to be cured.
I too was having problems with an old HP printer under Snow Leopard (DeskJet 970cxi) and came across this excellent page which has drivers for 776 HP and other printers. I simply installed the three files listed and the printer was working perfectly again!
I have the same problem with both my hp4100 and 4500. i know not the newest, but even the laserprint updates from HP does not do it for me. not even in my network. I have to open an other computer to be able to print in a network.
I have down-ed my snow to the normal leopard, until al my software is snow-ready.